Hume believes that the only meaningful thoughts are those about relations of ideas (known a priori, examples including mathematics and logic) and matters of fact (known a posterior, examples including 'the sun rises' and 'carrots are orange'). But Hume's psychology promotes the notion that thoughts have meanings only insofar as they are reducible to impressions, since according to Hume our ideas reduce to impressions. As such, why does Hume make an exception for mathematics and logic, holding them to be meaningful thoughts, even though they are not themselves reducible to impressions?